Which element from the following is radioactive?

Which of the following elements belongs to the inner transition elements?

$Ce - 58$ is a member of

The first element of the rare-earth metals is:

Consider the following statements:
$I$. $La(OH)_3$ is the least basic among hydroxides of lanthanides.
$II$. $Zr^{4+}$ and $Hf^{4+}$ possess almost the same ionic radii.
$III$. $Ce^{4+}$ can act as an oxidizing agent.
Which of the above is/are true?

Which element from the following has a half-filled $f$ orbital in the $+3$ oxidation state?